Biochar amendment holds promise for improving saline soils, yet its efficacy is often constrained by the uncertainty of application rates. In this study, a large field trial and associated statistical modeling were conducted to explore the mechanisms by which biochar affects wheat yield in coastal saline soils of northern China. Results showed that biochar application significantly increased soil organic carbon (SOC) content (R2= 0.615, p < 0.001) but induced marked spatial heterogeneity across the field, with the coefficient of variation (CV) reaching 30.2%. Given the difficulty of uniformly applying biochar in the field, subplot-level SOC was used as a proxy for effective biochar distribution. Stepwise regression identified soil electrical conductivity (EC) as the dominant yield constraint (standardized coefficient = −0.69), rather than water and nutrients, and a quadratic relationship was observed between SOC and EC. Structural equation modeling (SEM) further suggested a trade-off: SOC was associated with higher yield through reduced bulk density (BD) (path coefficient = −0.603), whereas high SOC levels were also associated with increased EC under this coastal saline field setting (path coefficient = 0.243), thereby indirectly constraining growth. Consequently, the agronomic response showed a threshold-like transition: the peak wheat yield occurred at an SOC threshold of 13.87 g kg−1 (equivalent to 44.41 t ha−1), which exceeded the point of minimum salinity (11.71 g kg−1, equivalent to ~29.90 t ha−1 biochar). These results suggest that the agronomic benefit of biochar in saline soils depends on maintaining application within an estimated beneficial buffering zone.

ABSTRACT Cover crops can enhance soil biological quality and increase soil organic carbon stocks. This study aimed to assess changes in microbial biomass, activity, and soil organic carbon stocks in cocoa farming systems in Ilhéus, Bahia, Brazil. Four cocoa farming systems were evaluated: full sun with Brachiaria cover crops, full sun with Fabaceae cover crops, full sun with spontaneous vegetation (natural), and full sun without any cover crops, arranged in a randomized completely block design. And an Agroforestry System (AFS) with cocoa trees intercropped with Eritrina velutina served as a reference. Soil samples (0–10 cm layer) were collected from the cocoa tree lines. Analyses included microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) via the fumigation-extraction method, soil microbial activity (SMA) based on CO2 evolution, soil organic carbon, and labile carbon through wet oxidation (SOC and LC, respectively), and total nitrogen (TN) using the Kjeldahl method. The study calculated soil organic carbon stocks (SOCstock), carbon loss rates, metabolic quotient (qCO2), and microbial quotient (qMIC). Results indicated an increase in MBN in treatments with cover crops compared to the control, suggesting that cover plants can improve microbial attributes in full-sun cocoa systems. However, MBC levels did not significantly change with cover crops, highlighting the need for longer cultivation periods to reach AFS levels. Notably, cocoa in full-sun with cover crops increased SOC in the 0–10 cm layer, with the Fabaceae consortium showing a similar SOCstock to the 40-year-old AFS. Thus, Fabaceae cover crops increase soil SOCstock in the short term after the change of land use.

ABSTRACT Soil salinisation poses a global threat to agricultural sustainability, affecting about one billion hectares of farmland. This review highlights integrated strategies—combining water management, agronomic practices, and biochemical interventions—to mitigate salinity while improving overall productivity. Precision irrigation methods, such as subsurface drip and microsprinklers, raise water‐use efficiency by 25%–40% and reduce surface salt buildup. Agronomic approaches—deep tillage, land levelling, and organic or inorganic amendments—enhance soil structure and increase soil organic carbon by 18%–32%. Biochemical tools, including salt‐tolerant germplasm and rhizosphere microorganisms (e.g., plant growth‐promoting bacteria and arbuscular mycorrhizal fungi), can boost yields by up to 50%. Unlike prior reviews focussing on isolated tactics, this work emphasises interdisciplinary synergies, such as subsurface drip irrigation creating favourable conditions for microbial inoculants and salt‐tolerant crops. It also addresses key socioeconomic barriers, including high initial costs and technical expertise gaps, and proposes future research on landscape‐scale modelling, circular resource use, and climate‐resilient saline agroecosystems.

To analyze the effects of in situ vegetable residue return on soil properties and microorganisms, this study conducted a continuous three-season in situ residue return experiment with four treatments: no return (CK), residue return (HTJ), residue return + compound microbial inoculant (HTJS), and residue return + ammonia water (HTJN). This study compared the treatment effects on soil quality. The results showed that, after the third tillage, the HTJS treatment increased soil organic carbon, total nitrogen, and mineralizable organic carbon content, and significantly enhanced the activity of soil β-glucosidase and soil peroxidase, which are related to carbon cycling enzymes compared to other treatments. There were no significant differences in bacterial or fungal α-diversity among treatments. Differences in fungal community soil β-diversity among treatments were significant. The HTJS treatment enriched organic matter-degrading bacteria Flavisolibacter and Devosia. Although HTJS increased the relative abundance of Fusarium, the field disease incidence index did not increase. The soil quality index (SQI), based on the minimum dataset (MDS), showed that HTJS had the highest SQI after the third tillage. Further path model analysis revealed that soil carbon components index and soil physicochemical index were the main controlling factors influencing the SQI. In conclusion, in situ residue return with a compound microbial inoculant (HTJS) is an effective strategy to simultaneously enhance soil fertility and biological activity by regulating the microbial community structure and associated enzyme activities.

Nitrogen (N) addition can increase soil organic carbon (SOC) in some grasslands, however, it is unknown whether N addition may enhance SOC in a warmer climate at broad spatial scales. We conduct a field experiment to test how N addition and warming interactively influence SOC in a temperate semiarid grassland. N addition significantly increases SOC by 14%, while simultaneous N addition and warming significantly decreases SOC by 25% relative to N-only addition. This result is further supported by a global meta-analysis, which shows that N addition (1 g N m-2 yr-1) increases SOC content by 0.2% relative to ambient conditions in grasslands, but the positive effect of N on SOC significantly declines at a rate of 0.06% per °C of increased mean annual temperature. Our field experiment and meta-analysis suggest that warming may reduce or even eliminate the positive effect of N addition on SOC in grasslands.

Treated wastewater enhances soil aggregate stability by increasing soil organic carbon: Evidence from δ13C and δ15N signatures.

From soil carbon towards system sustainability: Integrating SOC modelling and life cycle assessment to evaluate environmental trade-offs in carbon farming

Impacts of inorganic and organic fertilization on soil organic carbon and crop production: a meta-analysis

Conservation Agriculture (CA) is pivotal to achieve sustainable intensification, yet the global efficacy of its core practice, conservation tillage (CT), remains debated regarding the trade-offs between crop productivity and ecosystem services across diverse environmental contexts. Here, we conducted a second-order meta-analysis, synthesizing 69 published meta-analyses, to elucidate the context-dependent drivers regulating the "win-win" outcomes of CT. Globally, CT reduced greenhouse gas (GHG) emissions by 5%, increased soil organic carbon sequestration by 21%, increased soil fertility by 11%, and reduced soil erosion by 12%, all while maintaining crop yields comparable to conventional tillage. However, CT can also emerge as a partial trade-off between crop yields and ecosystem services, notably between crop yield and GHG mitigation. These trade-offs were strongly regulated by climatic and edaphic conditions as well as management intensity. For instance, strong synergies between crop productivity and multiple ecosystem services were more pronounced in (semi-)arid regions characterized by low temperatures and low precipitation, as well as in coarse-textured alkaline soils. Furthermore, integrating CT with residue retention and crop rotations maximized these synergies, mitigating potential yield penalties. Collectively, our synthesis demonstrates that context-specific refinement of CT implementation is essential to reconcile agricultural productivity with ecosystem services, thereby advancing climate-resilient agricultural systems globally.

Context Soil acidity, high soil strength, and poor subsoil structure are major constraints to crop productivity in coarse-textured and texture-contrast soils of southern Australia. These interacting limitations restrict root growth, reduce access to subsoil water and nutrients, and constrain yield potential in water-limited environments. While deep tillage and surface liming have been used to address individual constraints, their benefits are often short-lived and insufficient to overcome multiple subsoil limitations simultaneously. Aims This study aimed to evaluate the short- and long-term effects of soil profile re-engineering on soil physical, chemical, and hydrological properties, and to determine the persistence of these changes over four cropping seasons at two contrasting sites in Western Australia. Methods In May 2021, four soil re-engineering treatments were established at Bolgart (deep sand) and Meenar (loamy duplex): untreated control; deep loosening with lime; deep loosening with lime and clay; and deep loosening with lime, clay, and compost – all applied between 0 and 80 cm depth. Soil properties including soil strength, bulk density, volumetric water content, pHCa, soil organic carbon (SOC), and cation exchange capacity (CEC) were measured at establishment, three months post-treatment, and 4 years later (2024). Key results At both sites and sampling times, untreated soils had subsoil strength exceeding the critical 2.5 MPa threshold for root growth below 10-cm depth. All soil re-engineering treatments significantly decreased soil strength to well below this threshold and maintained these improvements over 4 years, despite partial recompaction. Soil strength increases between 2021 and 2024 were substantially smaller than typically reported following strategic tillage alone. At Bolgart, treatments incorporating clay and compost increased soil water storage in the 0–80 cm profile by up to 25 mm relative to the control, whereas at Meenar, greater water retention in the untreated subsoil reflected limited root access rather than improved water availability. Lime incorporation increased subsoil pHCa by 1.5–1.7 units – an order of magnitude greater than surface liming – raising pHCa above critical thresholds at 10–70 cm depth and maintaining these improvements over 4 years. Incorporation of compost and clay resulted in marked increases in SOC and CEC, improving soil buffering capacity, stabilising soil physical condition, and reducing the likelihood of re-acidification. Conclusions Soil profile re-engineering produced rapid, substantial, and persistent improvements in subsoil physical, chemical, and hydrological properties, with benefits maintained for at least four cropping seasons across contrasting soil types. Implications These findings demonstrate that soil re-engineering can overcome multiple interacting subsoil constraints simultaneously and provide a mechanistic basis for the large yield and water-use efficiency gains reported in Part II of this series. With the development of cost-effective machinery, soil re-engineering offers a promising pathway to sustainably increase productivity in water-limited, constraint-prone cropping systems.

Increasing Soil Organic Carbon (SOC), the largest terrestrial carbon pool, through proper land management has been suggested as a nature-based solution to mitigate climate change. In this context, it is important to understand the impacts of land transformations on regional SOC stocks. The study spatially analyzed the tree plantation expansion in Kerala, India, along with other land transformations in the last five decades and its effect on surface (0–30 cm) Soil Organic Carbon (SOC) density and stocks. This study adopted a machine learning-based predictive modelling approach by combining: (1) a detailed two-time period land use map separating major plantation types; (2) legacy soil data representing ground SOC measurements for each land use category; (3) other climatic, topographic and soil variables that affect the spatial variation of SOC, in order to spatially assess the changes in SOC stocks in Kerala due to land use changes over the last five decades (1972–2020). The study highlighted significant local hotspots of losses and gains that the traditional area-based methods do not fully capture. Interestingly, although there was a large increase in the area under tree cover in the last five decades, SOC gains in certain regions were compensated by losses in other regions leading to a very small change (~ 2%) in the overall SOC pool size. Land use and soil type were the most important predictors of SOC based on the developed Random Forest model. The findings highlighted that afforestation with tree plantations might not always lead to an increase in SOC stocks at regional scales. Its effect on SOC stocks varied by plantation type and previous land use. These implications must be considered while adopting climate mitigation strategies. Also, spatially explicit evaluation of various plantation types improves SOC source sink modelling and should be considered for preparing more accurate regional & national SOC inventories.

Eggplant cultivation faces major challenges from continuous cropping obstacles, which degrade soil health and limit sustainable production. Microbial inoculants offer a promising strategy for addressing such issues by modifying the soil environment and rhizosphere ecology. In this study, a field experiment was conducted to evaluate the effects of three bacterial inoculants, including Bacillus zhangzhouensis (BF1), Bacillus mobilis (BF2), and Zhihengliuella halotolerans (BF3), on soil properties, microbial community structure, and crop performance in a continuously cropped eggplant system. The results showed that three inoculants exerted strain-specific effects: BF1 significantly promoted eggplant vegetative growth and yield, increasing plant height by 32.1%, stem diameter by 28.7%, and total yield by 142.4% relative to the control; BF3 selectively improved fruit quality and soil nutrient status, elevating eggplant fruit total amino acid, soluble protein, and soluble sugar contents by 68.9%, 52.3%, and 41.2%, respectively, and increasing soil organic carbon (SOC), total nitrogen (TN), and available nitrogen (AN) by 13.73%, 18.03%, and 84.92% compared with the control. BF2 showed limited efficacy relative to the control. All inoculants enhanced the abundance of beneficial bacteria and reshaped the rhizosphere microbial community structure. The findings demonstrate the potential of strain-specific microbial inoculants to alleviate continuous cropping obstacles and promote sustainable eggplant production.

ABSTRACT Background Rapid expansion of opencast mining in developing nations is exerting profound pressure on land resources, with the extent of degradation constraining the effectiveness of topsoil replacement and organic amendments in restoring soil organic carbon (SOC) and nitrogen (N) pools, which are key for soil health and productivity. Aim This study assessed the potential of perennial legumes ( Leucaena leucocephala , Gliricidia sepium , Pueraria phaseoloides ) to restore the fertility of mined degraded soils. Methods An on‐site revegetation experiment with a control (un‐revegetated) treatment and selected legumes was established. SOC and total N were monitored prior to and 2 and 5 years after revegetation. Two and 3 years after revegetation, maize was cultivated to assess soil productivity under the influence of selected legumes. Results There was initial decline in SOC and total N at early stages of revegetation. However, significant increases were observed relative to the control with pronounced effects 5 years after revegetation. Highest changes in SOC (3.4–12.4 g kg −1 ) and total N (0.3–1.1 g kg −1 ) were observed under L. leucocephala . Growth of maize revealed significant improvement under P. phaseoloides , attaining highest plant height (≈80 cm) compared to the lowest (<30 cm) in the control treatment 3 years after revegetation. Despite significant increases in SOC and total N, plant growth was generally limited. Conclusions Although legumes showed strong potential in restoring soil fertility, utilizing their full potential requires adaptive establishment strategies, especially for L. leucocephala , and consistent management of P. phaseoloides to control its vigorous spread and maintain balanced ground cover.

Straw compost products are considered an excellent organic amendment for acidic soils, yet their effectiveness and microbial associations remain poorly understood. This study employed a pot experiment to evaluate the effects of straw compost products from six crops (corn, soybean, wheat, rice, peanut, and canola) on corn growth and nutrient uptake, soil physicochemical properties, and microbial community in an acidic red soil and examined how microbial community changes relate to plant performance. The results showed that straw compost products significantly enhanced corn growth and contents of nitrogen, phosphorus, and potassium in the aboveground tissues, except for wheat and canola straw. Compost products also improved availability of soil nutrients to varying degrees and affected the bacterial community structures in bulk and rhizosphere soils. There were significant differences in the improvement effects among straw types, with leguminous crops being better than cereal crops. Corn growth was closely correlated with increased soil organic carbon. The influence of the rhizosphere on bacterial communities was stronger than that of straw compost type. The dominant phyla Actinobacteriota and Patescibacteria were key bacterial groups positively associated with corn nutrient uptake in the rhizosphere. Compared to the bulk network, the rhizosphere microbial co-occurrence network exhibited higher modularity and a greater proportion of positive edges, suggesting a more cooperative interaction pattern. The influence of compost products might be associated with distinct nitrogen and phosphorus transformation pathways. Overall, this study clarifies the differential effects of straw compost products on acidic soil improvement and reveals strong associations between rhizosphere microorganisms and crop nutrient uptake.

Cropland soils are central to climate mitigation because they can store additional soil organic carbon while simultaneously emitting nitrous oxide and methane. Over the past decade, numerous field experiments and global syntheses have quantified how widely promoted practices—such as conservation tillage, residue retention, cover cropping, organic amendments and biochar, and water-saving irrigation in rice—shape these three components of the soil greenhouse-gas balance. This article presents a systematic tri-gas review, second-order meta-synthesis of recent field-based meta-analyses and large multi-site experiments to evaluate mitigation trade-offs and co-benefits across soil organic carbon stocks, nitrous oxide emissions, and methane fluxes in croplands. The compiled evidence shows that conservation tillage combined with residue retention frequently increases soil organic carbon in surface layers, but associated changes in nitrous oxide are small, inconsistent, or even positive in humid, fine-textured soils. Cover crops and diversified rotations generally deliver modest soil carbon gains and agronomic co-benefits, yet their effects on nitrous oxide range from mitigation to exacerbation, depending on climate, soil properties, species choice, and nitrogen management. In flooded rice systems, alternate wetting and drying and related water-saving regimes consistently reduce methane emissions and often maintain yields, but their impact on nitrous oxide is highly variable and sensitive to fertiliser timing and rate. Biochar and carefully managed organic amendments emerge as among the few interventions that frequently increase soil organic carbon while reducing nitrous oxide across a range of conditions, although their effects on methane are more context dependent. Overall, the synthesis confirms that no single practice provides universal “win–win” mitigation across all gases and regions. Rather, the greatest and most reliable climate benefits arise from management portfolios that combine carbon-building practices with explicit nitrogen management and, in flooded systems, tailored water control, thereby aligning soil-based mitigation with productivity and resilience objectives.

Increasing soil organic carbon (SOC) aims to increase and maintain soil quality for sustainable crop production and to achieve carbon removal targets. Agronomic practices are therefore needed to reduce carbon losses and increase SOC stocks, especially in deep soil layers, which promote long-term storage. Regenerative agriculture is an approach aimed at increasing soil quality for sustainable production and therefore should be suitable for achieving the required goals under organic farming conditions. We analysed SOC content down to a depth of 1 m and calculated the SOC stock based on bulk density after ten years of regenerative farming practices, i.e., reduced tillage, dead organic mulch, and high-quality yard waste compost application, in two organic field trials set up one year apart in Central Germany and we calculated the carbon (C) and nitrogen (N) input to the soil from organic amendments and from main crops and cover crops by applying C allocation factors for crop residues, roots, and rhizodeposition. C derived from crops was the main carbon input source over ten years. Increasing C input promoted an increase in the cumulative SOC stock down to 1 m. We observed greater SOC stocks dominated by topsoil changes with reduced tillage and compost application and with the combination of all practices (+ 16%) than in the control with conventional ploughing and no external carbon input while none of the farming practices affected the subsoil SOC stock. Mulch application had no effect at all on SOC stocks. Crop biomass contributed most C input. Farming practices, especially the combination of reduced tillage and compost application, enhanced topsoil SOC stocks and N but not subsoil C storage. Other farming practices and crop rotation adjustments must be identified to increase crop production as well as subsoil SOC stocks and promoting long-term C storage, e.g., by fostering deep-rooting crops and cover crops.

Ecosystem restoration is increasingly recognized as a sustainable climate change mitigation strategy, yet global estimates of its carbon sequestration potential widely vary. Modeling-based studies differ in assumptions over key restoration aspects, including restorable areas and restoration outcomes. Many assume recovery of carbon stocks to pristine levels, an expectation not supported by empirical evidence. They also focus primarily on forests and biomass, with limited attention to soil organic carbon (SOC). Here, we estimate the global SOC sequestration potential of forest and grassland restoration by combining current SOC levels on degraded land areas available for restoration with empirically derived SOC increase factors at the ecosystem scale. We provide spatially explicit estimates of SOC sequestration potential, absolute and per hectare. We also assess the carbon sequestration potential achievable under national forest restoration pledges across four major resolutions. With 1223 million hectares (Mha) of degraded land globally, the SOC sequestration potential is 38.5 GtC, of which 35.1 GtC (IQR 30.4–39.3 GtC) in forests and 3.4 GtC (IQR 2.6–4.2) in grasslands. National pledges cover 133 Mha, whose restoration could sequester 4–5.5 Gt of SOC. We show that there is a large unexplored theoretical climate mitigation potential of restoration globally. Environmental policies targeting Southeast Asia and South America, where potential is high and pledges are low, are particularly promising.

Increasing soil organic carbon (SOC) levels is essential for sustainable agricultural productivity and climate change mitigation, particularly in alkaline soils with inherently low SOC. While amorphous Al hydroxide (Am-Al) significantly influences SOC stabilization in volcanic and humid-region soils, and biochar enhances SOC in temperate and tropical regions, their effectiveness and stabilization mechanisms in alkaline soils require further exploration. We conducted a 1-year incubation of low SOC alkaline soils (5.2 g kg−1) amended with 13C-labeled maize residue (1 g kg−1), with or without Am-Al or rice-husk biochar (each 10 g kg−1); residue mineralization/retention was quantified and molecular composition profiled by solid-state 13C NMR and Py-GC/MS. Rapid decomposition of plant residue ceased around 12 weeks, while plant residue-derived C and native SOC decomposition continued throughout the incubation period. Am-Al significantly reduced maize mineralization within the initial two weeks and retained a higher proportion of residue-derived C than the control soil with maize addition after one year (Am-Al: 36% vs. Control: 28%). 13C NMR and pyrolysis–GC/MS showed smaller decreases in carbohydrate-C and saccharides and a higher carbon preference index and odd–even predominance of alkanes, indicating that Am-Al better preserved carbohydrate- and cuticular-wax-derived components, proxies for less-degraded residues. Respiration dynamics and molecular fingerprints indicate Am-Al rapidly stabilizes labile plant compounds, possibly through non-electrostatic sorption and ligand-exchange. Biochar also retained more residue-derived C (33%) than the control, but its effects on mineralization emerged later in the incubation (>6 months). We attribute this lag to surface degradation/activation of the biochar, which may stabilize residue-derived C more efficiently. Overall, adding Am-Al or biochar with plant residues significantly increased residue-derived C retention through immediate and delayed mechanisms, respectively. Treatments combining Am-Al or biochar with plant residue yielded a net positive C balance over the incubation, whereas residue alone was negative. Thus, the application of Am-Al and biochar with plant residues represents a promising strategy for sustained C stabilization, thereby improving SOC in degraded alkaline soils.

Potential of crop residues management for soil organic carbon sequestration in European countries until 2050: a simplified modelling approach

Bio-based fertilizers (BBFs) are gaining attention as sustainable alternatives to mineral fertilizers due to their potential not only to enhance soil health and crop productivity, but also to close nutrient cycling. Derived from urban, agricultural or industrial origin, BBFs vary widely in composition, and their long-term impact on soil health and crop productivity remains insufficiently understood. To fill this gap, we evaluated the long-term effects (> 20 years) of various BBF applications, including compost, sewage sludge, cattle slurry and human urine on soil physical, chemical and biological properties, comparing them with conventional mineral NPK fertilizer in the CRUCIAL field experiment in Denmark. Our results showed that while mineral NPK fertilizers support crop productivity and improve certain soil properties, they do not deliver the broader soil health benefits associated with organic amendments. Indeed, compost and sewage sludge notably increased soil organic carbon (SOC), cation exchange capacity, soil porosity, water content at field capacity and microbial activity, while reducing bulk density and clay dispersibility. Human urine exhibited a comparable nitrogen fertilizer effect in terms of crop yields to the NPK treatment, highlighting its potential for urban nutrient recycling. Although values for human urine treatment did not exceed any critical thresholds, a higher sodium adsorption ratio than mineral NPK treatment and a negative trend for bulk density were observed, indicating a need for complementary organic matter input. Multifunctional soil health assessments confirmed superior performance of organic amendments (compost and sewage sludge) across chemical, physical, and biological indicators, primarily driven by sustained organic matter inputs. These findings underscore the positive effect of long-term application of BBFs - especially compost and sewage sludge – not only on SOC increment but also on soil physical, chemical and biological properties that are essential for sustainable crop production. • Compost and sewage sludge improved several key soil health parameters. • Human urine resulted in highest sodium adsorption ratio. • Soil physical indicators for human urine application stayed below critical thresholds. • Carbon rich bio-based fertilizers decreased crop production per unit N. • Differences in input recalcitrance explained SOC productivity.